First step, take notes of what you currently have. Most plans have a bandwidth limit and this is the most important comparison part.
Then, with the info in hand, reach out to the other carriers, let them know what you have and ask if they can beat that plan. If they can, then go back to your current provider, and ask them to meet or beat the plan that you currently have, because their competitor has better pricing, or limits. Most of the time they like to keep current customers and they will give you some sort of compensation or adjust your plan to better suit you.
If they refuse to adjust your plan, let them know that you are activley seeking out better coverage, limits or a pricing model. I have asked for the customer retention team, and had quite a bit of success getting the companies to give customer adjustments.
Be positive and nice, most people will try to help and get you better deals!